You see error 0x800CCC78 when trying to send an email from Outlook. The error message states your sender address was rejected by the server. This happens because your outgoing mail server does not accept the email address you are using. This article provides the steps to resolve this authentication and configuration problem.
Key Takeaways: Fixing the Sender Rejected Error
- File > Account Settings > Server Settings: Verify your outgoing SMTP server address and authentication method match your email provider’s requirements.
- Outgoing Server (SMTP) Port 587 with TLS: This is the modern standard port for authenticated sending that most providers require.
- Account Settings > Change > More Settings > Outgoing Server: Check the box for “My outgoing server (SMTP) requires authentication” and select “Use same settings as my incoming mail server.”
Why Your Sender Address Gets Rejected
Error 0x800CCC78 is an SMTP authentication failure. Your mail server uses rules to verify you are authorized to send from a specific email address. When Outlook’s configuration does not meet these rules, the server rejects the connection. The most common cause is incorrect authentication settings for the outgoing SMTP server.
Many internet service providers and email services like Gmail or Outlook.com now require encrypted connections and explicit login for sending mail. This is to prevent spam. If your account settings still use an old configuration, such as port 25 without authentication, the server will block the send attempt. The error can also appear if you are on a new network, like a public Wi-Fi, that blocks standard SMTP ports.
Understanding SMTP Authentication
Simple Mail Transfer Protocol (SMTP) is the standard for sending email. Modern servers require you to prove your identity before accepting a message from your address. This is called SMTP authentication. In Outlook, this is controlled in the account’s advanced settings. The server compares the login credentials you provide with its list of authorized senders for your email domain. A mismatch causes error 0x800CCC78.
Steps to Correct Your Outgoing Server Settings
Follow these steps to update your account configuration and resolve the sending error. You will need your email provider’s correct SMTP server name and port number, which are usually found on their support website.
- Open Outlook Account Settings
Go to File > Account Settings > Account Settings. Select your email account from the list and click Change. - Review Server Information
In the Change Account window, verify the Your Name and Email Address fields are correct. Note the Outgoing mail server (SMTP) address. Click More Settings. - Configure Outgoing Server Authentication
In the Internet Email Settings window, go to the Outgoing Server tab. Check the box for “My outgoing server (SMTP) requires authentication.” Select the option “Use same settings as my incoming mail server.” - Set the Correct SMTP Port
Click the Advanced tab. Under Outgoing server (SMTP), change the port number. For most providers, use port 587. For the encrypted connection type, select TLS from the dropdown menu. Click OK. - Test the Configuration
Click Next in the Change Account window. Outlook will test your account settings. If the tests pass, click Close and then Finish. Try sending a new test email.
Alternative Method: Recreate the Account Profile
If correcting the settings does not work, the account profile may be corrupted. Create a new profile from scratch. First, note all your current server details. Then go to Control Panel > Mail > Show Profiles. Add a new profile and manually enter your account information with the correct SMTP authentication and port settings.
If Outlook Still Has Issues After the Main Fix
Error Persists After Updating SMTP Settings
If the error continues, your network or security software may be blocking the connection. Temporarily disable any third-party firewall or antivirus software to test. Also, try connecting your computer to a different network, like a mobile hotspot, to rule out network-level port blocking by your internet provider.
Outlook Asks for a Password Repeatedly
Constant password prompts indicate an authentication failure. Go to File > Account Settings > Account Settings, select your account, and click Change. Re-enter your password in the Password field. Ensure you are using an app password if your email account uses two-factor authentication.
Sending Works but Receiving Fails
This points to an issue with your incoming mail server (IMAP/POP) settings, which are separate from the SMTP error. Verify the incoming server address, port, and encryption method in the Advanced tab of your account settings. An incorrect incoming server will not cause error 0x800CCC78, but it will prevent you from getting new mail.
Manual Configuration vs. Auto-Discover: Key Differences
| Item | Manual Configuration | Auto-Discover Setup |
|---|---|---|
| Setup Process | User must manually enter server addresses, ports, and encryption types | Outlook automatically detects settings using the email address and domain |
| Error 0x800CCC78 Likelihood | Higher if details are entered incorrectly | Lower, but can fail if the auto-discover service is unavailable |
| Best For | Older email services, custom domains, or when auto-discover fails | Major providers like Microsoft 365, Outlook.com, and Gmail |
| Control Over Settings | Full control to specify exact ports and encryption | Settings are applied automatically by the provider’s protocol |
You can now send emails without the sender rejected error. Ensure your SMTP authentication is always enabled. For advanced security, consider using an app-specific password if your email provider supports two-factor authentication. A final tip is to regularly check your provider’s support site for any announced changes to their SMTP server addresses or required ports.